This invention relates generally to latches and more particularly to latch devices for doors of vehicles such as motor vehicles or automobiles.
As is known, a door latch device of a vehicle such as a motor vehicle can be actuated in unlatching operation by manipulation of outside and inside door handles of the vehicle. In many cases a knob for locking is provided and is manipulated for placing the latch device of a closed and latched door in a state wherein the device cannot be unlatched by manipulation of either door handle. The same knob is manipulated in reverse movement to unlock the device. In other cases, depending on the type of vehicle, locking and unlocking of the latch devices of the vehicle is carried out by electromagnetic driving devices of an electrical control circuit which can be controlled by the manipulation of a button or knob by the vehicle driver.
In a latch device of known type having an electromagnetic driving device, a shifting member in the form of a lock lever for locking operation is provided in the interior of the body of the latch device and is driven by the electromagnetic driving device coupled thereto in rotation between the lock state and the unlock state. These lock and unlock states are two completely different states, and an intermediate state between these two extreme states cannot be permitted.
Accordingly, in a known latch device an elastic force is caused to be imparted to the lock lever, which has the function of establishing the lock state and the unlock state, on either side of a dead-center point as a dividing point. For this purpose, a so-called over-center spring is installed to cause the lock lever to undergo an angular displacement with a click motion from either one of two terminal positions respectively of the lock state and the unlock state to the other terminal position, and the lock lever cannot stall or linger at any position other than these two terminal positions.
The over-center spring exerts a relatively great spring force, which retains the lock lever in either one of its two terminal positions and assures that it will not shift randomly to any other position. However, this means that this considerably great force of the over-center spring must be overcome up to the dead-center position in shifting the lock lever from one terminal position to the other.
Accordingly, in the case of a latch device of the type wherein the above described displacement of the lock lever is remotely controlled by means of a driving means such as a motor or a solenoid, a motor or solenoid of large size and high output must be used as the driving means. This gives rise to a problem since, in the case of a door latch device of a motor vehicle or the like, it is desirable that the driving means be one of small size and low output as much as possible because it must be installed within the interior of the door.